Typically, vehicles, such as cars, are equipped with airbag devices that restrain occupants by inflating and deploying airbags in the vehicles in case of a crash. Various types of airbag devices, such as driver airbag devices equipped in steering wheels, passenger airbag devices equipped in instrument panels, side airbag devices equipped in side panels or seats of vehicles, and curtain airbag devices equipped in areas above doors, have been developed and used. These airbag devices are normally stored in vehicle components so as to make their presence unnoticeable. In an emergency such as a crash, the airbag devices inflate and deploy airbags by making the airbags burst through vehicle components. Accordingly, while airbag-releasing portions of airbag devices normally need to be covered to conform to the appearance of interior equipment of vehicles, they must be able to smoothly and quickly release airbags in an emergency.
FIG. 5 is a sectional view of a passenger airbag device. The passenger airbag device is equipped in an instrument panel 91, on the vehicle body side, positioned in front of a passenger seat. As shown in FIG. 5, the passenger airbag device includes an airbag (not shown), which is normally folded and is inflated and deployed in an emergency, an inflator 92 for supplying the airbag with gas, a retainer 93 for housing the airbag and the inflator 92, an outer panel 94 connected to the instrument panel 91, and an inner case 95 connected to the inside of the outer panel 94, for holding the retainer 93. The retainer 93 is linked and fixed to a fixing member 97 through a connecting member 96.
The instrument panel 91 has an opening 91a through which the airbag is released. The outer panel 94 is fitted to the opening 91a with clips 94a or the like, and the surface of the outer panel 94 is finished so as to conform to the appearance of the instrument panel 91. In the inside of the outer panel 94 is formed a plurality of grooves 94b for enabling the outer panel 94 to be easily split when the airbag is deployed. These grooves 94b divide the outer panel 94 into fixing portions 94c and door portions 94d. 
The inner case 95 includes an inner panel 95a connected to the inside of the outer panel 94, and side walls 95b defining a space for housing the retainer 93. The side walls 95b include a plurality of hook holes 95c for receiving hooks 93a attached to the retainer 93. The inner panel 95a is separated into two at the center thereof, and each inner panel 95a includes a supporting portion 951 connected to the corresponding fixing portion 94c of the outer panel 94, a flap portion 952 connected to the corresponding door portion 94d, and a hinge portion 953 formed between the supporting portion 951 and the flap portion 952. As shown in FIG. 5, each hinge portion 953 is substantially U shaped, with the apex section of the hinge portion, the supporting portion 951, and the flap portion 952 being smoothly connected. A non-welded portion having a width w is left between the outer panel 94 and each hinge portion 953.
The above-described passenger airbag device activates the inflator 92 to supply the airbag (not shown) with gas in an emergency such as a vehicle crash. The airbag inflates in the retainer 93 and the inner case 95, and pushes the inner panels 95a and the outer panel 94. The outer panel 94 is split at the groove 94b between the door portions 94d, and the airbag is released in the vehicle. As the airbag is inflated and deployed, the door portions 94d in the outer panel 94 are turned toward the instrument panel 91, and further split at the grooves 94b between the fixing portions 94c and the door portions 94d. At this time, since the fixing portions 94c and the door portions 94d in the outer panel 94 are connected to the inner panels 95a, the door portions 94d do not fall away when split at the grooves 94b. The door portions 94d are further turned toward the instrument panel 91 by the action of the hinge portions 953 of the inner panels 95a until they have opened outward completely.
The inner case 95 is typically made of a resin and is easily affected by the ambient temperature to become hard or soft. Thus, to allow the door portions 94d in the outer panel 94 to properly open outward, the shape of the hinge portions 953 needs to be well designed. For example, a hinge portion disclosed in Japanese Unexamined Patent Application Publication No. 2006-96267 is formed such that it is curved in the direction away from an outer panel, and gradually becomes thin from a fixing portion side toward a door-reinforcing portion side (refer to Japanese Unexamined Patent Application Publication No. 2006-96267, FIG. 4). A hinge portion disclosed in PCT Japanese Translation Patent Publication No. 2003-137057 is formed such that the apex section of the curved portion is thinner than the other portions of the flexure hinge portion (refer to PCT Japanese Translation Patent Publication No. 2003-137057, FIG. 4).
However, since the hinge portion disclosed in Japanese Unexamined Patent Application Publication No. 2006-96267 is generally thicker on the fixing portion side, the amount by which it stretches when an outer panel opens outward is small. Thus, the size of the entire hinge portion (i.e., the diameter of the U-shaped portion) needs to be increased to compensate for the small amount of stretching. If the diameter of the U-shaped portion is increased, however, the width w of the non-welded portion (refer to FIG. 5) left between the outer panel and each inner panel increases, resulting in a problem in that the effect of the inner panels as reinforcements for the outer panel is reduced. In addition, if the size of the hinge portion is increased, the entire hinge portion is pushed during inflation and deployment of the airbag, resulting in another problem in that the door portions of the outer panel cannot smoothly open outward.
The hinge structure disclosed in PCT Japanese Translation Patent Publication No. 2003-137057 has a problem in that, since the portion which is most likely to be stretched when the door portions of the outer panel open outward (the apex section of the curved portion) is thin, the stress is concentrated too much on the apex section of the curved portion.